Abstract - The present study was carried out on a Palearctic-Indian migratory species, the blackheaded bunting (Emberiza melanocephala), to understand the importance of photoperiodism and circannual rhythms in determining seasonality in changes in body mass
and testis size in birds. An initial experiment determined the effects of duration and intensity of light on photoperiodic
induction. The birds were exposed to different photoperiods (hours of light:hours of darkness; 11.5L:12.5D, 12L:12D, 12.5L:11.5D
and 13L:11D) at the same (~ 450 lux) light intensity, and to 13L:11D at different light intensities (50-, 100-, 400-, 800-
and 1000-lux). The induction and subsequent regression of photoperiodic responses were dependent upon duration and intensity
of the light period until these reached threshold. A second experiment investigated if an endogenous seasonal rhythm underlies
photoperiodism in buntings. Birds maintained since February on a 8L:16D photoperiod (a non-inductive short day length invariably
used to ensure photosensitivity in photoperiodic species) were subjected periodically to 16L:8D (a long day length), one group
every month from mid-March to mid-August. The magnitude of long day response in body mass and testes decreased as the duration
of the short days progressed, but testicular response was restored in birds that were exposed to long days in July and August.
The birds exposed simultaneously to short, long, and natural day lengths for 32 weeks underwent an induction-regression cycle
under long days and natural day lengths, but not under short days in which a decrease in body mass occurred after about 20
weeks. The last experiment examined the importance of latitudinal migration on photoperiodism, by comparing the response to
long days of three groups which included birds from populations those were held in the outdoor aviary for 1 or 2 years at
27° N and those immediately arrived from their breeding grounds (~ 40° N). There was no difference in the photoperiodic induction
among the three groups, indicating that neither experience to changing photoperiods during a migratory journey, nor to long
photoperiods at breeding grounds, were critical for a subsequent response (initiation-termination-reinitiation) cycle. Taken
together, these findings suggest that (1) the blackheaded bunting has its own endogenous timing program, which is regulated
by the photoperiod, and (2) the photoperiodic programs of bunting are flexible enough to accommodate variations in the amplitude
of environmental cycles. Thus, it appears that photoperiodism has evolved independently of the evolution of migration in this
species.